Public-key and certificate authentication are combined into the public-key authentication method.

The most commonly used user-authentication methods are password, public-key, and host-based authentication. In public-key authentication, the users upload their public key files to the server and edit a configuration file. The server thus has a database of user public keys, similar to the client having its database of server public keys.

When certificates are used in user authentication, the user does not need to upload any public key files to the server prior to the connection, and the server does not need to have a database of user public keys or certificates. The server validates the user certificate by using the CA certificates that the server has been configured to trust and authorizes the login based on the user certificate contents.

Keyboard-interactive is not an authentication method in itself, but more like a common interface to various other authentication methods that are based on keyboard input. Password authentication, RSA SecurID, PAM (Pluggable Authentication Module), and RADIUS are examples of authentication methods that can be used over keyboard-interactive.

The highest security is achieved by using token-based certificate authentication where the certificate and the private key are stored on a cryptographic token, such as a smart card. Secure Shell supports also several other strong authentication methods, including the proprietary RSA SecurID.

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